(1) Field of the Invention
The present invention relates generally to the field of boiler water level detectors and, more particularly to systems for the detection of electrical faults, as well as water versus steam detection, in boiler water level detectors.
(2) Description of Related Art
The control of water level in a steam boiler is extremely critical, in that if a fire tube is exposed, a boiler explosion could result. The rate of thermal conductance from a fire tube by water is many times greater than that of steam, thus if steam blankets the tube, there is insufficient heat conduction from the tube and the tube rapidly overheats. In order to avoid this potentially catastrophic occurrence, steam systems are required to provide redundant water level indicators for each boiler.
One common solution to the requirement for redundant water level indicators has for some time included two independent visual level indicators. Such visual level indicators require penetrations for top and bottom of the indicators and a sight glass in which the level of water is visually apparent. However, boilers typically operate at high temperatures and pressures, often at a normal operating pressure of 3,000 psi. This factor presents an engineering challenge to make the sight glasses robust in such a hostile environment. As a consequence, sight glasses often leak, and a small leak at such a high pressure can quickly develop into a serious problem.
In recognition of this problem, standards setting organizations, such as for example the ASME, have provided alternative standards for providing redundant level indications. For example, one such redundant visual indicator may be isolated and an electronic level indication used instead. Such electronic level indications must themselves be robust and ideally are redundant, typically parallel circuitry, and not scanned, polled or multiplexed systems, but fully independent systems. However, electronic level indicators present challenges in design all their own due to the high temperatures involved. A typical electrical-type level indicator uses a water column system with spaced-apart conductive electrodes (also referred to as probes), with the water column mounted directly to the side of the boiler. An electrical potential is impressed across each of the spaced-apart electrodes, and the current is measured. If water is present between the conductors, the low impedance of the water results in a relatively high measured current. Conversely, steam is much less conductive, and thus if steam is present between the conductors, the high impedance results in a relatively low measured current. Commonly the higher current is used to illuminate one color or type of lamp, and the lower current illuminates another color or type of lamp, thus presenting to an operator an easily visible level indication. Additional logic circuitry may provide steam over water error indication, high and low level trip points, alarms, pre-alarms, displays, relay closures, SSR conduction, and other indications.
Some such electrical level indicators use a low DC potential with an impressed square wave periodic signal impressed upon it. The DC potential may cause hydrolysis of the water, and can result in explosive concentrations of hydrogen and oxygen within the steam system or electroplating. Thus, there remains a need for an electronic level indication system for boilers that avoids the hydrolysis problem.
Further, standards setting organizations have mandated various self-diagnostic features, such as for example power supply monitoring, clock function monitoring, electrode short detection, open wiring detections, and water over steam detection. Many systems have been offered to serve these various functions, but there remains a need in the art to provide an integrated electronic system which can effectively detect a circuit short, a circuit open, and/or a water vs. steam condition, wherein the electronic system sensing circuit has no direct current component which may cause hydrolysis.
Martin el al., in U.S. Pat. No. 4,020,488, described an apparatus for indicating when the value of a parameter lies above or below a predetermined limit. The apparatus includes two or more sensors each arranged to provide a warning signal when the parameter lies beyond a respective specified value, the value for at least one of the sensors being equal to or beyond the predetermined limit; and logic means including a validation circuit connected to the two sensors and arranged to provide a xe2x80x9cparameter-beyond-limitxe2x80x9d signal. This signal may be used to operate an alarm and/or an emergency trip, only when both sensors provide a warning signal. Various forms of apparatus are described in which the parameter sensed is the level of water in various items of a steam plant as used in power stations to provide an indication of high and/or low water level.
Bartles, in U.S. Pat. No. 4,224,606, describes a fluid level control circuit with a safety test function for a boiler water level probe of a fail safe type. The control circuit is connected to a field effect transistor controlled amplifier having a well defined threshold of operation. A safety test switch is provided to pull the amplifier""s input down below the threshold point, but not to a shorted condition. By observing the output of the system, it is possible to determine whether the fluid level detector is properly installed, and whether the amplifier has had any type of unsafe operating failure.
Spencer, in U.S. Pat. No. 4,482,891, describes a robust electronic liquid level gauge apparatus, particularly suitable for use with boiler drum water column. The apparatus comprises simple probe sensors in the water column, differential amplifier sensors, detectors responsive only to the simultaneous presence of a minimum level signal and a certain frequency, and logic circuits continuously processing outputs from four detectors: its associated probe detector, the one below, and the two above.
Cosser, in U.S. Pat. No. 4,646,569, describes a fluid level measurement system for the measurement of a level of fluid in a vessel. The electrical impedance of a first conduction path between a central rod electrode and a surrounding cylindrical electrode and the electrical impedance of the second conduction path between the cylinder and the vessel are established. The first conduction path is restricted to be below the minimum fluid level by an insulating coating on the rod. The second conduction path varies with the fluid level. An output signal is extracted from the electrodes potentiometrically which is indicative of the fluid level. The signal may be linearized by application to a circuit having a matched non-linear transfer function. With this arrangement, a continuous output signal indicative of the fluid level is produced which is not subject to errors due to changes in fluid resistivity.
Colley et al., in U.S. Pat. No. 4,692,591, describe a controller for controlling an electrode boiler steam humidifier, of the type operable from a source of polyphase alternating current. A plurality of current loop transformers, coupled to the boiler electrodes, sense the individual currents drawn in each phase of the polyphase alternating current. A plurality of full-wave bridge rectifiers individually couple the secondaries of the current loop transformers to the input of a voltage amplifier such that the output of the amplifier is indicative of the highest of the currents sensed in each phase of the boiler electrode current. When the output of the voltage amplifier exceeds a predetermined threshold, a control effect is produced such that operation of the humidifier is controlled in accordance with the highest of the individual phase currents drawn through each of the boiler electrodes.
Similarly, Colley et al., in U.S. Pat. No. 4,792,660, describe a controller for controlling an electrode boiler steam humidifier. The controller includes a plurality of current loop transformers, coupled to the boiler electrodes, for sensing the current drawn by the electrode boiler. A current level detector provides a control voltage to a solenoid controlled valve, associated with the boiler, to drain a predetermined quantity of water from the boiler and thereby reduce the electrode current, when the electrode current exceeds a predetermined upper current limit. A resettable counter is provided for limiting the number of automatic attempts which can be made to reduce the electrode current. Normal periodic actuation of the humidistat resets the counter to avoid reaching the predetermined count through long-term acquisition of isolated over-current occurrences. During normal operation of the boiler, the time required for the electrode current to decrease between a pair of predetermined current levels is monitored and is compared against a known reference in order to provide a general indication of the condition of the boiler.
Jordan et al., in U.S. Pat. No. 5,519,639, describe a system for monitoring the water level of a pressure drum having a pressure column. The monitoring system comprises a plurality of electrodes communicating with the column for contacting water and steam within the column. Each electrode has its own location and produces a first output which corresponds to the presence of water or a second output which corresponds to the presence of steam. A discriminator is operatively connected to the electrodes and the discriminator has an analog-to-digital converter for receiving the output of each electrode and converting the output to a digital signal. The digital signal represents the water conductivity for each electrode. A central processor is connected to the discriminator for powering the discriminator as well as for receiving the digital signals in order to determine a slope degradation between the conductivity and the location of each electrode in the column. The central processor also determines an inflection point between the water and the steam in the column. An LED display is used to indicate which of the electrodes are located in water and which of the electrodes are located in steam.
Richards et al, in U.S. Pat. No. 5,565,851, describe a fluid level sensing system, particularly for sensing the water level in a steam generating boiler. The sensing system comprises a vessel connected to the boiler so as to contain water and steam at substantially the same level as in the boiler, a number of vertically spaced electrodes projecting into the vessel, and circuitry for measuring the electrical impedance sensed by each electrode. The circuitry, which can be multiplexed to each electrode in turn, includes a first comparator for producing an output when the sensed impedance lies between the normal maximum impedance of the water and the normal minimum impedance of the steam above the water, this output being demultiplexed to drive a vertical display to show the level of the steam/water interface. The circuitry additionally includes a second comparator for producing a further output when the sensed impedance lies below the normal minimum impedance of the water, which further output is indicative of a variety of different fault conditions and drives a fault indicator.
And finally, Richards, in U.S. Pat. No. 6,118,190, describes a circuit for controlling a fail-safe operation of measurement and control apparatus for detecting the presence or absence of water by making a measurement of the impedance experienced in a gap between an insulated tip of the electrode and a surface held at a reference voltage or else connected to ground. The apparatus may be configured to provide an alarm when water is present and should not be, or vice versa. The circuit comprises comparators, a phase detector, and a triple-redundant drive circuit which drives a relay. One comparator ensures that when operating with water as the normal condition, an electrode fault, such as could be caused by excessive contamination, will cause the system output to indicate an abnormal condition. Another comparator discriminates between the water condition and the steam condition. Yet another comparator ensures that there is no output if there exists an abnormal condition or any fault condition. It follows that there will only be an output if the sensor is in the normal condition. This is then shown to operate a relay using a secure triple redundancy drive circuit so that the relay contacts may be used to indicate a normal or an abnormal/fault condition.
However, the art does not describe a system which includes a dual frequency sensing signal which develops no direct current aspect, and which is used to indicate an open circuit condition, or short circuit condition on the same sense line, as well as boiler level measurements and a fault indicator, all within the same sensing circuit.
It is therefore an object of the present invention to sense for an open and/or a short circuit condition, as well as a water vs. steam condition with a single system. It is a further object of this invention to provide a method of sensing for an open and/or a short circuit condition, and a water vs. steam condition, with a single circuit. It is also an object of this invention to provide for sensing such conditions with a circuit with a net sum integral zero signal applied to the electrode probes to eliminate the possibility of hydrolysis within the boiler system.
The present invention addresses these and other needs in the art by providing a dual-frequency signal generator which develops two AC components with no associated DC component. The two frequencies are mixed and sent through an impedance matching circuit to match the impedance of the signal generating portion of the system with the impedance of the boiler water under measurement.
The impedance-matched signal is then directed to two legs, one leg directed through one of a plurality of electrode probes and then to a first filter circuit, and the other leg is directed a second set of filters. In combination, the filters pass either the higher or the lower of the two frequencies to determine an open or short condition in the level sensing circuitry, as well as a steam over water condition.